Nitrogen is needed in large amounts by plants as a mineral nutrient. Plant growth, particularly in agriculture, is often limited by the amount of available nitrogen.
Traditionally, plants have been considered to use only ammonium and/or nitrate as sources of nitrogen. These compounds are supplied either by natural processes, such as the mineralisation of organic nitrogen and nitrogen fixation by symbiotic prokaryotes, or by the application of fertilisers containing industrially fixed nitrogen.
However, recent research has indicated that plants may also have access to certain organic nitrogen compounds, as well as inorganic nitrogen (Näsholm, T. et al. (1998) Nature 392, 914-916, Näsholm, T. et al (2000) Ecology 81: 1155-1161, Näsholm, T. and Persson, J. (2001) Physiol Plant 111: 419-426, Lipson, D. and Näsholm, T. (2001) Oecologia 128: 305-316). In particular, the uptake and metabolism of amino acids from soil seems to be a ubiquitous feature of plants.
Among the amino acids used as building blocks for proteins, all but one (glycine) can be found in two isomeric forms which are distinguished by their ability to rotate polarised light. The form found in largest quantities in nature rotates light to the left and is termed L- or levorotatory, while the less common form is termed D- or dextrorotatory.
Whilst D-amino acids are found in nature, they are present only at very low concentrations and only in specific compounds as cell wall proteins in bacteria (e.g. in the compound peptidoglucan).
Although plant amino acid transporters mediate transport of both D- and L-forms of amino acids (Soldal, T. and Nissen, P. (1978) Physiol. Plant. 43: 181-188, Boorer, K. J. et al. 1996. J. Biol. Chem 271: 2213-2220, Montamat et al (1999) Plant Mol. Biol. 41: 259-268), plants lack the necessary enzymes to convert D-amino acids into nitrogen forms that can be used in synthetic reactions inside the plant. Plants cannot therefore use D-amino acids as a source of nitrogen.
The capacity to metabolise D-amino acids is, however, widespread among bacteria, fungi and animals. Several reactions have been described, leading to catabolism of D-amino acids in organisms (see FIG. 2) (Friedman, M (1999) J. Agric. Food Chem. 47: 3457-3479, Pilone, M. S. (2000) CMLS 57: 1732-1747, Yurimoto et al. (2000) Yeast 16(13): 1217-1227).